1. Field of the Invention
The present invention generally relates to location of obscured conduits, and more particularly to a method of, and apparatus for, determining the direction to and position of an underground cable or pipeline which is capable of carrying an alternating electrical current.
2. Description of the Prior Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, power and television. It often becomes necessary to locate defective or damaged cables, pipes, etc., in order to repair or replace them. Conversely, it is important to know the approximate vicinity of such items in order to avoid disturbing them when digging or excavating for other purposes.
A convenient method for locating underground telephone or electrical cables exists in the prior art. Typically, the cable comes to the surface at various spaced locations in terminal boxes known as pedestals. An amplified signal source may be directly connected or inductively coupled to a given wire or wire pair at the pedestal. The wire acts as an antenna, re-radiating the signal along the full length of the cable. A receiver unit may then be used to trace the path of the cable above ground. While this technique is highly suited to electrically conductive (current carrying) wires, it may also be used on conductive pipes. Moreover, nonconductive conduits may be installed with a parallel wire marker which can be used to trace the path of the conduit.
The present invention relates to the receiver unit used to detect the electromagnetic radiation emitted by the conduit or marker. Several instruments have been devised which perform this general function. For example, U.S. Pat. No. 4,387,340 issued to E. Peterman discloses a receiver having four sensors mounted in a fixed relationship. Two of the sensors (electrical pick-ups or antenna coils) are used for left-right determination of the position of the buried conductor. In operation, the axes of both coils are horizontally disposed and the coils are horizontally spaced. The outputs are subtractively combined to produce a signal which is related to the difference between the magnetic flux in each coil. Thus, when the device is centered over the conductor, the field strengths through the coils cancel each other out, which is reflected in an appropriate display device. When the difference is non-zero, the phase of one horizontal coil is compared to the phase of the other to determine leftright position. The identical invention is also disclosed in a divisional application, now U.S. Pat. No. 4,520,317.
A device operating on a slightly different principle is described in U.S. Pat. No. 4,427,942 issued to L. Sole. That device employs two coils whose axes are horizontally disposed, the coils being separated vertically. The coil signals are rectified and the average DC voltages compared. When the apparatus is directly over the conductor, the two signals are essentially equal (nonzero), and an indicator alerts the user to this fact. This device does not, however, provide left-right guidance.
Another invention operates on a phasecomparison principle similar to Peterman. U.S. Pat. No. 4,639,674 depicts an apparatus having three coils, all vertically disposed, lying in a triangular formation, the plane formed thereby being vertical also. A flip-flop serves as a phase detector between the two signals from the lower coils. The third coil acts to eliminate the effect of ambient magnetic fields.
Three other patents of interest employ phase comparison to locate the conductor. U.S. Pat. Nos. 4,134,061 issued to H. Gudgel; 4,438,389 issued to A. DeSa; and 4,390,836 issued to Bruce et al. each include additional coils for locating faults ("holidays") in the conductor in addition to tracing the conductor path. The '836 patent is of particular interest inasmuch as it employs two orthogonally disposed coils for tracing the conductor path.
The use of orthogonally disposed coils is also illustrated in U.S. Pat. No. 4,220,913 issued to Howell et al. Four coils are shown in a rectangular arrangement; however, only the two vertically disposed coils are initially used to determine whether the unit is directly above the buried conductor. Left-right determination is then made by comparing the phase of the signal from one of the vertical coils to the phase of the signal from one of the horizontal coils. In other words, it is not necessary to use all four coils for left-right determination (the "extra" coil is used for depth determination).
Another orthogonal configuration of coils is disclosed in U.S. Pat. No. 4,542,344 issued to Darilek et al., which is probably the closest prior art. As shown in FIG. 1A, that invention includes two sensors 10 and 12 horizontally separated a distance R, which are held a fixed distance from the ground by a rod 14. Each sensor comprises two coils, one vertically disposed and one horizontally disposed, resulting in four leads A, B, C and D. As further illustrated in FIG. 1B, inputs from these leads are directed to a multiplexer 16 which combines the signals into a single output to an automatic gain control (AGC) 18. AGC 18 averages the amplitude of each signal against the prior four signal amplitudes, which is passed to demultiplexer 20 to yield weighted voltage values (V.sub.A, V.sub.B, V.sub.C and V.sub.D) for each coil. These values are then used to compute the leftright offset L according to the equation: ##EQU1## This computation is made in microprocessor 22, and passed to display 24.
The primary disadvantage of the foregoing devices is their limited accuracy. Several of the devices provide left-right indication by means of an audible signal, requiring that the technician be familiar with the characteristic tones of the unit. This subjective factor introduces human error into the system, which is exacerbated by interference from ambient electromagnetic fields. Accuracy is further impeded by variations in the received signal amplitudes and additional conductors buried in the vicinity. For example, all of the foregoing devices have difficulty in resolving two adjacent, parallel cables carrying the same AC signal. This effect is especially pronounced when current flow in one cable is opposite that in the second cable. It would, therefore, be desirable and advantageous to devise an apparatus which sharply discerns lateral crossover as the unit traverses a buried conductor carrying an alternating current.
Accordingly, the primary object of the present invention is to provide a method and apparatus for locating buried conductors.
Another object of the invention is to provide such an apparatus which detects electromagnetic radiation emitted from the buried conductor in such a manner as to clearly distinguish the point of lateral crossover.
Still another object of the invention is to provide a locating apparatus which gives an indication of proximity of the conductor as well as indicating whether the conductor lies to the left or to the right of the user.
Yet another object of the invention is to provide such an apparatus which is portable and easy to use in field applications.